Endotracheal tubes intended for insertion through the mouth, nose or implanted in the neck (oro-naso and tracheostomy tubes) are usually provided with an inflatable cuff for sealing against the tracheal wall. The efficiency of the sealing is determined by the magnitude of the cuff pressure against the tracheal wall since the inflated cuff does not seal off pressures exceeding the pressure of the cuff against the wall. The air pressure in the cuff determines the pressure against the tracheal wall. The pressure of the cuff against the tracheal wall can be controlled and regulated only if the cuff has a sufficiently large diameter to make contact with the tracheal wall without stretching of the sheet material of the cuff, i.e. the cuff must be lying folded on the tracheal wall. If this demand is met, the pressure in the cuff is identical with its pressure on the wall.
If the pressure of the cuff against the tracheal wall is considerably higher than 30 cm H2O, the blood supply to the mucosa is occluded, and this causes damage in the form of superficial and deeper ulcerations after some time. This damage is prevented in that the sealing cuff, lying folded on the wall, is kept inflated from an outer source with a constant, regulated pressure of 20 to 30 cm H2O.
The sealing cuff has the additional function of preventing liquid (blood, saliva, vomit) from flowing past the cuff down into the lungs. It has been found that this function is accomplished when the pressure of the sealing cuff against the tracheal wall is at least 20 to 30 cm H2O.
Spontaneous changes in the diameter of the trachea, changes in the catheter position and the diffusion of certain anaesthetic gases through the wall of the sealing cuff may cause considerable changes in the pressure in the sealing cuff if the pressure is not controlled and regulated.
During artificial respiration the necessary pressure of the respiration air may often exceed 20 to 30 cm H2O, and the pressure in the sealing cuff without a valve is then too low to seal off the pressure of the respiration air.
In order for the sealing cuff to be able to seal off high inflation pressures during artificial respiration, the respiration catheter described in DK-C-111149 is equipped with a two way cut-off valve mounted on the air supply tube for sealing the cuff. Due to the flow resistance between the two ends of the catheter and consequent difference in pressure, the valve closes prematurely and opens prematurely. This entails that part of the air in the sealing cuff escapes during the expiratory phase so that, for a short moment, the cuff can not provide an efficient sealing of the trachea.
The respiration catheter described in U.S. Pat. No. 5,497,768 has a cut-off valve outside the cuff in the vicinity of the free end of the catheter. The valve membrane is an elastic membrane along the circumference of the catheter covering the inlet and the outlet of the cuff inflation tube. When, during inflation of the lungs, the pressure in the airway downstream in relation to the cuff exceeds the pressure in the cuff inflation tube, the valve is in the closed position and prevents air from escaping from the cuff. When the pressure in the airway decreases below the pressure in the cuff inflation tube during expiration, the valve is in the open position and allows the pressure in the cuff to equilibrate with the outside pressure source. The potential risk inherent with the design is the valve membrane being disengaged and lost into the lungs.